netfilter: nf_tables: nat expression must select CONFIG_NF_NAT

This enables the netfilter NAT engine in first place, otherwise
you cannot ever select the nf_tables nat expression if iptables
is not selected.

Reported-by: Matteo Croce <technoboy85@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>